You tried, directly or indirectly, to change the value of a
constant. This type of error frequently occurs at a distance and is difficult to trace. The magic of $_ is often involved, but is not responsible.

Modifying elements of @_ directly allows you to
modify the variable that was passed to the function. For example, in the above example $n is now 2. But an error will occur when a constant is passed, as in the second call.

The variables $a and $b are aliased to each item in the list being sorted, and as such modifying them is possible - but will cause an error if the current element is unmodifiable. A common cause of this is sorting an array of references when where the list has a gap. In this situation $a will be undef, and autovivification by dereferencing will trigger an error.

This example will cause an error because the for loop aliases $_ to the literal '1', and then calls prompt_user which attempts to read a line from STDIN and store it in $_ - which is still aliased to '1'.

When putting a smiley right before a closing parenthesis, do you:

Use two parentheses: (Like this: :) )
Use one parenthesis: (Like this: :)
Reverse direction of the smiley: (Like this: (: )
Use angle/square brackets instead of parentheses
Use C-style commenting to set the smiley off from the closing parenthesis
Make the smiley a dunce: (:>
I disapprove of emoticons
Other